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Infrared Limb Sounding With Cassini CIRS



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Infrared Limb Sounding With Cassini CIRS Optimal Viewing Strategy Using Horizon Nodes Conor A Nixon Richard K Achterberg Department of Astronomy University of Maryland College Park MD 20742 301 286 6757 301 286 1550 conor a nixon nasa gov richard k achterberg nasa gov F Michael Flasar Solar System Exploration Branch NASA Goddard Space Flight Center Greenbelt MD 20771 301 286 3071 f m flasar nasa gov Abstract In this paper1 2 we investigate a question of science optimization during Cassini flybys of Titan The Composite Infrared Spectrometer CIRS makes limb observations along an atmospheric path above surface during the closest approach period when the visible horizon circle is moving swiftly across the planet We have sought to discover if any points on the horizon are preferred for limb sounding due to having minimum movement relative to the surface By numerical calculation backed by geometric analysis we find that two limited regions on the horizon are continuously visible during the entire encounter We term these limb nodes and show how they may be employed by CIRS to optimize science by minimizing the source of systematic error due to spatial smear These conclusions are applicable to many similar scenarios of spacecraft limb sounding during hyperbolic flyby encounters a substantial atmosphere During these close approaches Cassini s Composite Infrared Spectrometer CIRS instrument is used to sense the atmosphere in limb sounding mode viewing along a path that does not does intersect the surface to measure the vertical variation of temperature aerosols hazes and gas composition The opacity of a limb path defined as TABLE OF CONTENTS When viewing the limb the available choice of latitude and longitude locations is defined by the instantaneous horizon circle At large distances D an entire hemisphere is visible and the horizon is a great circle circumference of the globe However as the spacecraft approaches to distances where the range is no longer much greater than the



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